The concentrations of many elements, including boron, are increasing in aquatic ecosystems due to anthropogenic activities. Laboratory studies were undertaken to evaluate the toxicity of waterborne boron to two key freshwater macroinvertebrates and to evaluate the effects of water hardness and sulfate on boron toxicity. Acute toxicity evaluations of waterborne sodium tetraborate resulted in 48 h. LC50 values of 141 and 1376 mg B/L for neonate Daphnia magna and fourth instar Chironomus decorus, respectively. Chronic sublethal studies demonstrated a significant decrease in C. decorus growth rate at 20 mg B/L. Further studies showed that increasing water hardness (10.6 to 170 mg/L as CaCO3) and sulfate (10.2 to 325.4 mg SO4(-2)/L) concentrations did not affect boron toxicity to D. magna. These results, in conjunction with a review of the literature, suggest that aquatic macrophytes may be more sensitive to boron than macroinvertebrates and thus would be better choices for aquatic risk assessment evaluations for this element.